Here are some realistic cumulus-fractus clouds. I've been attempting these for ages now and couldn't get the control I was after. Sometimes you just need to leave an idea for a while and go back to it after a new wave of thinking has had time to creep up and make things all clear.

The following variation progression image is the rendered results of moving only one slider upwards, between '0' and '1' and hitting render.

These are simply rendered in direct lighting. Of course, they are far more flexible than this...The cloud layer is fed by two fractals, these are split into two main shader channels for dual-control of the cloud input; 'Form and Seed' and 'Wisp Dominance'.Pretty self explanatory. The form/seed fractal describes the overall shape of the cloud layer, wisp dominance is how much of this form is 'eaten' and puffed by turbulent wispy noise.

To use them, simply insert them as a clip file into a scene, connect them as you would any other cloud layer, random seed the form/seed fractal until you get a nice overall shape for your layer and then just begin to raise or lower their wispiness. Use the 'coverage' slider in the wisp dominance layer to add/remove their fractus factor. It's set to default for '0.25' wispiness coverage, a nice happy level. Altitude constraints can also be set in this layer for the wispiness.

If you don't like the pattern of the wispiness but still want to keep the main form of cloud you've got, just change the 'wisp fractal' seed/noise flavour/warp etc. for different variations.

In the clipfile version remember to check the 'blend by shader' boxes of each node the 'distance shader' is plugged into, and attach your render camera to it as well. The distance shader blending is unused in the default clip. It's there if you do want it, though, ready to be enabled.

The cloud layer requires only 47 samples at full quality detail.

That's about it, maybe someone can light them up nicely, I wan't to see some nice red sky decay on the feathers of these birdies! I'm away to play with these myself now.